mbox: Fixed assert-crash with read-only mbox streams (e.g. zlib plugin)
mbox: Fixed assert-crash with read-only mbox streams (e.g. zlib plugin)

--HG--
branch : HEAD

jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jwm/dovecot-notmuch